As the complexity of photonic integrated circuits (PICs) increases, the physical size and the number of electrical connections, among others, also increase significantly. Unlike electrical integrated circuits, PICs comprising at least one optical waveguide require one or more optical interfaces in order to get photons in or out of the PIC, i.e. to enable optical communication between said at least one optical waveguide and an external optical system such as an optical fiber.
Regarding packaging of an optical subassembly that comprises a PIC and an external optical system, the optical interfaces between the PIC and the external optical system present several constraints.
A first exemplary constraint is the use of a lens or an optical coupling system to maximise the coupling efficiency between the PIC and the external optical system.
A second exemplary constraint is that it is absolutely essential to fixate the external optical system, e.g. an optical fiber, at the point of highest coupling efficiency from the PIC to the external optical system, i.e. to align and maintain alignment of the first longitudinal central axis of the first optical waveguide of the PIC and the second longitudinal central axis of the second optical waveguide of the external optical system.
A third exemplary constraint is that a method of aligning the external optical system to the PIC and fixating the external optical system to the carrier substrate must be automatable for use in high-volume low-cost applications. Furthermore, the method must use low-cost materials which are commonly available.
Several processes are in common use to fixate an optical fiber in place to within 500 nm precision to provide a stable optical interface between the PIC of an optical subassembly and the optical fiber output. These processes include the application of unfilled resins or resins filled with particulate matter to reduce shrinking of the resin upon curing. Both the unfilled and the filled resins are curable by at least one of ultraviolet (UV) light and heat. A further process includes fixation of optical fibers using metal clips or preforms, either welded or induction heated.
Currently, all of the processes mentioned above have main disadvantages. Unfilled resins or epoxies have an inherent problem with shrinkage during curing, often 2% to 5% shrinkage of linear dimensions. A glue line between the optical fiber and the carrier substrate of the optical subassembly may be 100-150 μm thick. In this case, a linear shrinkage of 2% to 5% percent may yield an alignment drift of 2 μm to 7.5 μm during curing. It is known that as a result of the latter significant alignment drift, coupling efficiency between the first optical waveguide of the PIC and the second optical waveguide of the optical fiber will drop at least by 5 dB. This may be countered by intentionally mis-aligning the fiber to account for the shrinkage induced alignment drift, but such a process is not repeatable enough for use in production environments.
Filled resins or epoxies comprise uncompressible particles, e.g. acrylate spheres having a diameter in a range of about 1 μm to 5 μm. Upon curing a filled resin, the liquid resin part exhibits the same linear shrinkage of 2% to 5% as in the case of unfilled resins. This shrinkage, in turn, causes the solid particulate spheres to pack tightly together. The compound shrinkage effect of a filled resin is then minimized to well below 0.5%. However, the filling particles clog automated dispensing nozzles and therefore this process is not suited for being automated in a production environment. Regarding the application of filled resins, the cost of labour is prohibitive.
Optical fiber fixation using metal harnesses is a scalable and automatable process. However, the biggest drawback is the need for per-design micromachined metal harnesses to fit both the optical fiber and the geometry of the packaging for the optical subassembly. The requirements of per-piece parts and a specialized laser welding setup add a large cost to the overall packaging total. The bill-of-materials for this packaging method alone can be two orders of magnitude higher than the resin- or epoxy-based methods mentioned above.